A light emitting diode (LED) often can provide light more efficiently than other types of light sources, such as incandescent or fluorescent light bulbs. The relatively high power efficiency associated with LEDs has created an interest in using LEDs to displace conventional light sources in a variety of lighting applications. For example, LEDs can be used in traffic lights, cell phone keypad and display illumination, etc.
An LED is a semiconductor device that emits light when electrically biased in a forward direction. The light emission is a form of electroluminescence. An LED may generally include a chip of semiconducting material doped with impurities to create a pn junction. When forward biased, electrons are injected into the junction from the n-region and holes are injected from the p-region. The electrons and holes release energy in the form of photons as they combine. The emitted photons can be visible as light to an observer. The wavelength of the light can depend on the bandgap energy of the materials forming the pn junction.
Recently various advances have been made in terms of semiconductor materials used in LEDS. The materials can provide increased efficiency and operation across a wide variety of wavelengths. However, despite advances in materials used, much of the light emitted from the semiconductor material is lost due to internal reflections and absorption of the emitted light within the LED device. Many semiconductor materials have a high index of refraction as compared with the index of refraction of air at a semiconductor-air interface, leading to significant internal reflection. A high efficiency LED can be one in which a particular geometry is chosen which allows a greater emission of light from the device.
Previous solutions have used surface texturing to increase light output. Surface texturing uses a chemical etching process to provide texture to the surface of an LED device. However, controlling the shape and feature sizes of the texture can be difficult. In some devices a Distributed Bragg Reflector (DBR) has been added to the LED to reduce light absorption in the semiconductor substrate. However, DBRs are only able to reduce light absorption over a limited light incidence angle. In other devices, a photonic crystal has been added to the emitting surface of the LED to diffract wave-guided modes out of the device. However, the fabrication of the photonic crystal on the emitting surface can be complicated and the end result provides increased efficiency over only a limited light bandwidth and the diffracted light is not quite collimated. A red LED with Lambertian transmittance and reflectance surfaces has been developed, but this LED uses multilayer structures with an undulating surface and a graded refractive index. These features increase fabrication complexity and cost and still cannot prevent light at oblique incident angles from being lost.